Thursday, August 4, 2011

Glaze Chemistry Simplified and Demystified

I have a lot of people ask me about my glaze process. Every listing in my Etsy shop mentions that my glazes are "handmade," but what exactly does that mean? In order for pottery to be food safe the surfaces which will come into contact with food must be sealed. 

Pottery is sealed with a very unique material which works directly with the clay, adapting, expanding and shrinking right with the clay body during the kiln firing. This material, known as a glaze is not a special type of colored paint. It is actually a very specific combination of materials which when mixed together create a wide range of surface options, offering the potter a world of visual, tactile and functional surfaces!

After reading through a few different books in an attempt to find a simplified, though thorough explanation of what exactly makes a glaze a glaze I happily stumbled upon this succinct sentence: "Any glaze fired at any temperature, is a combination of three components: Fluxes, Amphoteric (Alumina), and Glass-formers." (From Robin Hopper's The Ceramic Spectrum.) These materials are sold in dry powder form. Following a formula the powders are mixed, water is added and a mesh screen is used to ensure fine, even particle size through out the mixture.

The flux in the glaze is the base, the alumina is the neutral and the glass-former is the acid. The flux or base is the material in the glaze that dictates when the glaze will melt. I now fire at cone 8, meaning my firing temperature reaches 2280 degrees Fahrenheit. The flux in my glaze allows my glaze to melt at this desired temperature, not well before or after. In order to get the glaze to stick to the pot the glaze must have a clay based, or alumina material added to it, this is the neutral in the glaze. It helps the glaze to stay on the pot and not slide off. And, finally the acid or glass-former is added to the glaze to give the glaze the glassy surface most of us are familiar with when we think of the word "glaze". In most cases this glass-former is silica, as it is in the two glazes I use most often.

Once these basic materials have been tested, explored and altered to fit the needs of the firing temperature and type of kiln, other ingredients are added to customize the glaze further. Colors are produced in a glaze through the use of metals in some form. I generally use Stains and Oxides in my glazes to achieve the desired color.

Currently I am working with a selected palette of Mason Stains which I blend to create the exact shade I envision. I also frequently use common colorant additives such as Copper Carbonate, Chrome, Tin, Iron and Rutile. My present glazes are primarily high gloss, translucent, colored glazes. However, if I wanted to create an opaque buttery surface I could (and likely will some day very soon), with a basic understanding of the materials I would need to add and change in my formula.

There are commercial glazes on the market that are widely used instead of these handmade glazes. The commercial glazes follow this same principle. I have tried both, but for what I am trying to accomplish with my work nothing compares to a handmade glaze. Handmade glazes allow me to study the molecular chemistry behind my glazes and with the right information, change my surfaces completely, nearly on a whim. I enjoy having my hands in as many aspects of my work as possible. Creating custom glazes to complement my wheel-thrown porcelain allows me to feel completely creatively fulfilled! 

Mixing glazes in my home studio means mixing glazes in my kitchen. This may be messy, but the results are worth it. I will never go back to commercial glazes. If you have considered mixing your own glazes but are not sure where to begin the following books have been incredibly useful in the development of my personal glaze knowledge: Most recently, Robin Hopper's, The Ceramic Spectrum. And a longtime favorite, Ron Roy and John Hesselberth's, Mastering Cone 6 Glazes. The recipe below is the Glossy Clear Liner Glaze from Mastering Cone 6 Glazes:

G-200 Feldspar:   20*
Ferro Frit 3134:  20
Wollastonite:     15
EPK:              20
Talc:              6
Silica:           19


* I have fired this glaze to cone 8 with no change from the cone 6 recommended temperature. I have tested this glaze with Minspar substituted for the G-200 Feldspar with success. I have also substituted the G-200 with Custer Feldspar, this version I found to be ever so slightly off-clear, but it is still a wonderful starting base glaze for testing colorants! Happy glaze making!